It has been a desired goal in the field of radar technology for many years to extend the data processing capability of the radar system to the point wherein the characteristics of the target can be used as an identification function. However, with the dispersion of identical types of military equipments to many countries which are now friendly, but could easily someday in the future become hostile, simple basic parameter identification of a target of interest may not be sufficient to accomplish an identification task. It can easily be projected that the time may come when positive identification is an absolute necessity. The anticipation of this potential requirement, and, in conjunction with work currently under way in identifying objects by their characteristic, FM modulations of the impinging transmitted signal by the targets surface vibrations has led to consideration of a system of coded vibration. Thus we not only examine the basic vibrations of a target but we can establish a coded vibrating system that will positively identify the target.
The problem of target detection has compassed many modes of identification using an array of parameters and processing schemes. The U.S. Air Force realized many years ago that they had an absolute requirement for positive aircraft identification. The aircraft have had a number of different transponder type schemes that require radiating sources and expensive equipment that requires sophisticated maintenance. It has been discovered that the identification process can be done on a cooperative passive basis using simple techniques with a minimum hardware sophistication and expense.
The discovery of a cooperative passive target technique is basically an offshoot of work dealing with detection of target vibrations. Once the basic concept of mechanization is understood for detection of target surface vibration, the next step is to try to create a controlled vibration system that can involve optimizing deflection of moving elements to maximize the FM sideband structure created by the surface movement. For example, it can be shown that the maximum first order sideband occurs when the ratio of signal wavelength to target displacement amplitude is approximately seven (7). However, under this particular condition, sidebands out to tenth order and beyond have to be considered in detecting the vibration effects on the impinging signal. Optimizing therefore implies more than designing the vibration deflection magnitude for maximum sideband generation.